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Understanding the downconversion photoluminescence of NaLaP4O12:Er3+ phosphor
Ceramics International 45 (2019) 20988–20993
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Short communication
Understanding the downconversion photoluminescence of NaLaP4O12:Er3+ phosphor
T
Yang Zhoua, Xinchen Gea, Zhuohui Zhanga, Wenjie Luoa, Hang Xua, Wufu Lia, Jing Zhua,b,∗ a b
School of Materials Science and Engineering, Yunnan University, Kunming, Yunnan, 650091, PR China Yunnan Key Laboratory for Micro/nano Materials & Technology, Kunming, Yunnan, 650091, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metaphosphate Phosphor Photoluminescence White LEDs
We have employed the solid-state chemical reaction to construct series of trivalent erbium ions activated NaLaP4O12 (abbreviated as: NLP) phosphors. The structural investigation indicates that NLP:Er3+ crystalline products are purely monoclinic phase with micron particles. Trivalent erbium ions with the substitution for lanthanum are successfully inserted into NLP host lattice. The UV–Vis diffuse reflectance absorption performance, doping Er3+ content-dependent downconversion photoluminescence, content quenching mechanism were reported in this paper. Upon near UV (378 nm) excitation, yellow/yellow-green visible light with the dominant 4S3/2 → 4I15/2 emission of trivalent erbium is displayed. Yellow-green emitting NLP:2%Er3+ sample has the maximum intensity and main luminescent peaks at 541 and 549 nm. The content quenching is related to the dipolar interaction. Additionally, the detailed investigation on CIE chromaticity coordinates, CP and CCT reveals that the title product has promising application in the field of white LEDs excited by near UV light.
1. Introduction Sodium lanthanum metaphosphate NaLaP4O12 (NLP) materials was early reported [1,2]. It is a monoclinic phase with P21/n space group, and the basic structural units are [(PO3)4]4– chains and LaO8 polyhedron. However, in recent years, considerable research has supported the importance of NLP-based phosphors activated by rare-earth (RE) ions (RE = Ce, Pr, Nd, Eu, Tb and Dy) [3–6]. NLP has the characteristics of facile synthesis, environmental protection as well as favorable crystalline field for typical narrow emission lines of RE activator ions. For example, Liu et al. developed NLP:RE3+ (RE3+ = Eu3+, Tb3+) phosphors via the solid-state chemical reaction [6]. Owing to the large distance and weak interaction between RE ions in NLP host lattice, content quenching phenomenon of Eu3+ or Tb3+ is not observed in NLP. Upon vacuum ultraviolet (UV) light (172 nm) excitation, Eu3+/ Tb3+-codoped NLP phosphors display the Tb3+ → Eu3+ energy transfer and Eu3+ content-dependent tunable visible light emitting from yellowish-green to reddish-orange, indicating the promising applications in field emission display (FED) and plasma display panel (PDP). However, we have not checked the optical report on Er3+-activated NLP materials to this day. Among RE ions, trivalent erbium ion is an effective activator for downconversion (DC) and upconversion (UC) photoluminescence, because its abundant 4f emission lines cover the UV, visible and near ∗
infrared (NIR) light band. Several illustrations (LaBMoO6:Er3+ [7], NaLuCrF4:Er3+ [8], Sr2Gd8(SiO4)6O2:Er3+ [9], SrGdGa3O7:Er3+ [10], Y2Mo4O15:Er3+ [11] and Gd2O2S:Er3+ [12]) show the multifunctional applications of Er3+-activated DC and UC emitting materials. For instance, BaTiO3:Er3+ green emitting phosphor reported [13] can be promisingly applied to the display and temperature sensing domains. Upon excitation at both 380 nm UV light for DC and 980 nm laser for UC, there is green luminescence performance with emission peaks at 550 (4S3/2 → 4I15/2 of Er3+) and 525 (2H11/2 → 4I15/2). Another example is that Er3+-activated telluroborate glass may be employed as potential materials for broadband optical amplifier and green laser [14]. However, compared with UC photoluminescence research, there is little research on DC photoluminescence of trivalent erbium. In aspect of white light emitting diodes (LEDs), inserting RE ions into host matrix is a key design strategy for different visible light emittings [15–19]. Our research aim is to acquire new phosphor with superior luminescence, facile synthesis and environmental protection. Herein, in this paper, we constructed a series of NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples via the solid-state chemical reaction. We have systematically researched the crystalline phase composition, particle morphology, ionic states, absorption performance, Er3+ content-dependent DC photoluminescence behavior, fluorescent quenching mechanism, color coordinates, correlated color temperature and color purity. Our work shows that yellow-green emitting NLP:2%
Corresponding author. School of Materials Science and engineering, Yunnan University, Kunming, Yunnan, 650091, PR China. E-mail address: [email protected] (J. Zhu).
https://doi.org/10.1016/j.ceramint.2019.06.268 Received 2 June 2019; Received in revised form 24 June 2019; Accepted 26 June 2019 Available online 27 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 20988–20993
Y. Zhou, et al.
Er3+ powder is a prospective phosphor component for white LEDs excited by near UV light. 2. Experimental details 2.1. Synthesis Series of targeted NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples were constructed via the solid-state chemical reaction. Analytical reagents Na2CO3, La2O3, NH4H2PO4 and Er2O3 (99.99%) were purchased as starting materials. The solid-state chemical reaction is illustrated as following:
(1/2)Na2CO3 + [(1 − x)/2]La2O3 + 4NH 4 H2 PO4 + (x/2)Er2O3 700 °C ⇒ NaLa1 −x Erx P4 O12 + (1/2)CO2 + 4NH3 + 6H2 O
(1)
The synthesis steps can be detailedly described as below: Firstly, based on the stoichiometric ratio of chemical formula, all starting materials were accurately weighed, fully mixed and ground in an agate mortar. Then the mixture was packed into a corundum crucible and preheated at 400 °C for 3 h in the air atmosphere. The preheated mixture was again thoroughly ground. The next step was to raise the temperature to 700 °C and sinter for 7 days by several intermediate grindings. In the end, the targeted sample was cooled to room temperature, pulverized again into white powder and collected for the following characterizations. 2.2. Characterizations For all as-prepared samples, after drying at 333 K, we have performed all measurements at room temperature. Powder X-ray Diffraction (PXRD) for the phase purity identification was performed using a diffractometer (Rigaku D/Max-3B) with Cu Kα radiation (λ = 1.54056 Å), 40 kV and 40 mA. A spectrometer (Thermo Scientific Nicolet iS 10) was employed for fourier transform infrared (FT-IR) analysis (The background is KBr pellet). The surface morphology information was obtained by a scanning electron microscopy (SEM) (Quanta 200 FEG), and the elementary composition was analyzed using the microscope with an energy dispersive X-ray (EDX) detector. An electron spectrometer (Thermo fisher Scientific K-Alpha+) with Mono Al Kα (1486.6 eV) excitation was used for X-ray photoelectron spectroscopy (XPS) analysis. A UV/VIS spectrophotometer (SHIMADZU UV2600) was employed for UV–Vis diffuse reflectance spectrum (DRS). A spectrophotometer (Edinburgh, FLS920) with xenon lamp excitation was used for photoluminescence excitation (PLE) and photoluminescence (PL) data. 3. Results and discussion 3.1. PXRD and FT-IR studies
Fig. 1. PXRD patterns for NLP as the function of Er3+ content (x).
trivalent erbium ion with eight-coordinated environment is close to that of trivalent lanthanum. In addition, the ionic radii of sodium and phosphorus are much smaller than the above radius data [20]. Hence, doping trivalent erbium replaces lanthanum with the same oxidation state, and the crystalline structure of NLP-based samples is unchanged in spite of the introduction of Er3+ ions. The FT-IR spectra for pure NLP and NLP:2%Er3+ samples are illustrated in Fig. 2. The two FT-IR curves display a series of similar vibration peaks (including relative intensities, positions and shapes) between 1400 and 400 cm−1. The vibration bands/lines in the 1310–1150 and 1150–1000 cm−1 regions result from the antisymmetric and symmetric stretching modes (νas and νs) for (O–P–O)– groups in the same order. The vibration bands/lines in the regions of 1000–850 and 850–620 cm−1 are associated with the νas and νs modes for (P–O–P) bridges in the same order. In the low frequency range below 620 cm−1, the vibration patterns result from the bending modes (δ) of (O–P–O)– groups and (P–O–P) bridges. However, it is difficult that the antisymmetric and symmetric bending modes (δas and δs) are clearly distinguished owing to the overlap with external vibrations [21,22]. Therefore, for Er3+-doped samples, [(PO3)4]4– chain in NLP host matrix is not destroyed. The FT-IR investigation is well comparable to the PXRD results (Fig. 1). 3.2. Surface morphology and elemental analysis Fig. 3 is the SEM micrograph for NLP:2%Er3+ powder sample. The crystalline particles are on the uneven micron scale. The observed nonregular shape results from the grain aggregation. This aggregation phenomenon is widespreadly present in the solid-state chemical reaction. The micron scale of as-synthesized particles is similar to that of commercial phosphor for LEDs [23]. The inset of Fig. 3 is the EDX
The phase purity of all as-prepared NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples was checked by PXRD technology, as depicted in Fig. 1. The bottom of Fig. 1 is the simulated PXRD reflection of NLP compound via the Visualizer software based on the structural data (ICSD code: 415682) of crystallographic information file (CIF) reported previously [1]. The diffraction data of NLP compound is also obtained and marked as JCPDS 36–0113. So the comparison between the experimental and simulated diffraction data/JCPDS 36–0113 was carried out, and all PXRD patterns stay the same in the measured range from 10 to 90°. All the as-synthesized NLP:Er3+ powder samples crystallize in the monoclinic structure of NLP host matrix (lattice constants: a = 7.2655, b = 13.1952, c = 10.076 Å, β = 90.38°, V = 965.96 Å3 and Z = 4) [1]. From the view of the crystalline coordination in NLP, the effective radius of trivalent lanthanum ion with eight-coordinated environment is 1.16 Å [20]. The effective radius (1.004 Å) [20] of 20989
Fig. 2. FT-IR spectra for NLP host and NLP:2%Er3+.
Ceramics International 45 (2019) 20988–20993
Y. Zhou, et al.
3.4. Downconversion photoluminescence properties of erbium
Fig. 3. SEM image for a typical sample NLP:2%Er3+. The inset shows the EDX spectrum.
spectrum for NLP:2%Er3+ powder sample. Typical peaks for erbium, lanthanum, phosphorus and oxygen are observed without impurity source. The conductive adhesive employed for the EDX measurement results in the presence of carbon peak. The chemical valence states for these ions are investigated by XPS study (Fig. 4). The reference value for the calibration of XPS data is the binding energy (284.4 eV) of C 1s peak. From the wide scan XPS spectrum for NLP:2%Er3+ powder sample (Fig. 4(a)), there are characteristic peaks for Er 3d, Er 4d, O 1s, P 2p, La 3d and Na 1s. It keeps consistent with the elemental composition in the inset of Fig. 3. The high resolution scan XPS spectrum (Fig. 4(b)) for P 2p consists of two peaks as P 2p1/2 (134.62 eV) and P 2p3/2 (133.98 eV), originating from the O–P – O bond of [(PO3)4]4– chains in NLP host matrix. The oxidation state of phosphorus ion is +5 [24]. For O 1s of O2− ion (Fig. 4(c)), the lattice oxygen (including P–O – P, Na–O, La–O or Er–O bonds) is associated with O1 (531.12 eV) and O2 (532.89 eV) peaks, and chemisorbed oxygen gives rise to weak O3 peak at 535.07 eV [25]. The binding energy (1070.56 eV) of Na 1s peak supports the fact that the oxidation state for sodium ion is +1 in the Na–O bond (Fig. 4(d)) [26]. For La 3d, La 3d5/2 results in two peaks at 834.59 and 837.59 eV, whereas La 3d3/2 contributes to two peaks at 851.4 and 854.9 eV. The result shows that the oxidation state of lanthanum ion is +3 (Fig. 4(e)) [27]. For Er 4d (Fig. 4(f)), the peak at 170.76 eV corresponds to Er 4d, suggesting the +3 oxidation state of erbium. It keeps consistent with the binding energy of trivalent erbium in erbium oxide [28,29]. The XPS result supports the fact that trivalent erbium ions are successfully introduced into NLP host lattice.
3.3. Absorption performance The room temperature UV–Vis DRS for NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples are presented in Fig. 5. For pure NLP host matrix, the cutoff edge of UV absorption originating from the host is located at around 310 nm, and visible light absorption is not observed. For all Er3+-doped samples, the magnified absorption part between 350 and 850 nm is the inset of Fig. 5. Some UV–Vis absorption peaks with identical position and shape are centered at 363 (4I15/2 → 4G9/2), 378 (4I15/2 → 4G11/2), 407 (4I15/2 → 2H9/2), 442 (4I15/2 → 4F3/2), 451 (4I15/ 4 4 4 4 2 4 2 → F5/2), 488 ( I15/2 → F7/2), 522 ( I15/2 → H11/2), 541 ( I15/2 → 4 4 4 4 4 S3/2), 652 ( I15/2 → F9/2), 663 ( I15/2 → F9/2) and 800 nm (4I15/2 → 4 I9/2), which result from the intrinsic 4f – 4f transitions of Er3+ [10,30]. The 4I15/2 → 4G11/2 (378 nm) absorption is the strongest. The absorption performance also suggests that the Er3+ activator ions are successfully inserted into NLP host matrix.
For NLP:2%Er3+ powder sample, the PLE (λem = 541 nm) and PL (λex = 378 nm) spectra are illustrated in Fig. 6. For PLE (λem = 541 nm) (Fig. 6(a)), there are some excitation peaks at 363 (4I15/2 → 4G9/2), 378 (4I15/2 → 4G11/2), 405 (4I15/2 → 2H9/2), 442 (4I15/ 4 4 4 4 4 2 → F3/2), 451 ( I15/2 → F5/2) and 468 nm ( I15/2 → F7/2), corresponding to the characteristic 4f – 4f transitions for Er3+ ion [31]. The obtained excitation information keeps consistent with the UV–Vis absorption performance (Fig. 5). The predominant excitation line is centered at 378 nm in near-UV region, indicating that the commercial nearUV LED chip can irradiate the synthesized powder sample for downconversion photoluminescence performance. For the PL (λex = 378 nm) spectrum (Fig. 6(b)), the typical 4f – 4f transitions of Er3+ ion results in three emission components [32,33]. The first emission part consists of several weak peaks at 450, 457, 462, 467, 473, 481 and 492 nm, which are related to the intrinsic transitions of trivalent erbium from the excited state 4F7/2 to ground state 4I15/2. The second includes two peaks at 523 and 531 nm, which result from the 2H11/2 → 4I15/2 transitions. The third (4S3/2 → 4I15/2) is predominant, and strong peaks are located at 541 and 549 nm. Fig. 7 depicts the Er3+-doped content-dependent PL (λex = 378 nm) spectra of NLP:Er3+ powder samples. All the PL spectra exhibit identical emitting peaks. For the strongest 541 nm emission (the inset of Fig. 7), Er3+-doped content-dependent emission intensity shows that the emission intensity increases when enlarging the Er3+-doped content (< 2 mol%), whereas it decreases when the doping content is larger than 2 mol%. Hence, NLP:2%Er3+ sample has the maximal 541 nm emission intensity. The Er3+-doped content quenching phenomenon gives rise to the PL result [34]. For NLP:Er3+ powder samples, to understand the content quenching mechanism, the formula (marked as (2)) reported by Blasse [35] is used for computing the critical energy transfer distance (Rc) for erbium ions.
R c ≈ 2(3V/4πχ cN)1/3
(2)
where χc is the Er3+-doped critical content, V is the volume per unit cell, and N is the substitution position number inside each cell. According to the structural data (ICSD code: 415682) of crystallographic information file (CIF) for NLP crystal reported previously [1], V and N is 965.96 Å3 and 4 in the same order. χc is equal to 2%. The calculated Rc is 28.5 Å. For the exchange interaction, the Rc is about 5 Å. Therefore, the content quenching phenomenon for NLP:Er3+ is mainly ascribed to the multipolar – multipolar interaction between Er3+ ions. We further research the multipolar energy transfer mode by the below equation [36]:
I/ χ= K[1 + β(χ)θ/3]−1
(3)
where K and β are constants, χ is the Er -doped content, and the emission intensity is marked as I. The different values of θ correspond to the different interaction mechanism. Since β(χ)Ɵ/3 is much greater than 1, the above formula (3) can be simplified to the below expression [36]: 3+
log (I/χ) = K, − θlog (χ )/3
(4)
From Fig. 8, the correlation of log (I/χ) and logχ is linear with the slope (−θ/3) value of −1.82. So the obtained θ equals 5.46, which is close to 6. Therefore, the dipole – dipole interaction is the content quenching mechanism of the as-prepared samples. 3.5. CIE chromaticity coordinates, CCT and CP To exactly determine the visible light color, correlated color temperature (CCT) and color purity (CP), we have speculated these performance indexes on the basis of the PL spectra of NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%), as shown in Table 1. The calculated CCT values for all as-synthesized samples range from 4325 to 5124 K. As drawn in Fig. 9, when the doping Er3+ content increases from 0.5 to 2%, CIE
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Fig. 4. Wide scan XPS spectrum (a) for a typical sample NLP:2%Er3+ and XPS spectra for P 2p (b), O 1s (c), Na 1s (d), La 3d (e) and Er 4d (f).
chromaticity coordinates (x, y) change from the yellow to yellow-green light in the standard CIE diagram. The emitting light changes towards the yellow light region when increasing the doping Er3+ content from 3 to 5%. The detailed CP values for all as-synthesized samples have been computed by the below formula [37]:
CP =
(x−xi)2 + ( y− yi)2 / (x d − xi)2 + (yd − yi)2
(5) 3+
Fig. 5. UV–Vis DRS curves for NLP as the function of Er3+ content (x). The inset shows the magnified absorption curves between 350 and 850 nm.
where the CIE chromaticity coordinates of NLP:Er are expressed as (x, y), the color coordinates of the dominant emission (541 nm) are marked as (xd, yd), and the chromaticity coordinates (0.33, 0.33) corresponding to standard white light are marked as (xi, yi). The calculated CP for visible light emitting of NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%) are more than 90%. The above results indicate that the as-prepared yellow-green emitting NLP:2%Er3+ sample is a prospective ingredient for white LEDs excited by near-UV light. 4. Conclusions On conclusion, erbium-activated sodium lanthanum phosphate (NLP) powders were developed using the solid-state chemical reaction. 20991
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Fig. 6. PLE (λem = 541 nm) and PL (λex = 378 nm) spectra for a typical sample NLP:2%Er3+.
Fig. 7. Er3+-doped content-dependent PL (λex = 378 nm) spectra for NLP:xEr3+ samples (x = 0.5, 1, 2, 3, 4, 5 mol%). The inset shows the Er3+ content-dependent PL intensity for 541 nm emission.
Fig. 9. CIE chromaticity image for NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%) samples under 378 nm excitation.
The micro-sized samples are NLP-based monoclinic phases. Yellow/ yellow-green visible light with dominant 4S3/2 → 4I15/2 is acquired upon 378 nm (4I15/2 → 4G11/2) excitation. The 2 mol% content of erbium ions is the optimum proposal for yellow-green emitting. The dipole – dipole interaction is the content quenching mechanism of the as-prepared samples. Yellow-green emitting NLP:2%Er3+ powder with high CP is a prospective phosphor component for white LEDs excited by near-UV light. Fig. 8. The curve of log (I/χ) vs. logχ for NLP:Er3+.
Acknowledgements Table 1 CCT, CIE chromaticity coordinates and CP for NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%) samples under 378 nm excitation. NLP:xSm3+
CCT (K)
CIE (x, y)
CP (%)
x = 0.5% x = 1% x = 2% x = 3% x = 4% x = 5%
4499 4561 5124 4611 4612 4325
(0.3740, (0.3718, (0.3471, (0.3696, (0.3906, (0.3854,
98 92 94 94 96 95
0.4394) 0.4420) 0.4197) 0.4416) 0.4612) 0.4580)
This research was financially supported by the National Natural Science Foundation of China (21761034), the Natural Science Foundation of Yunnan Province (2018FB017), and Undergraduate Innovation and Entrepreneurship Foundation of Yunnan University (201810673044 and 201804002).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.06.268.
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emitting diodes, ACS Photonics 5 (2018) 3801–3813. [19] Z.Y. Wang, H. Jiao, Z.L. Fu, Investigating the luminescence behaviors and temperature sensing properties of rare-earth-doped Ba2In2O5 phosphors, Inorg. Chem. 57 (2018) 8841–8849. [20] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A: Found. Crystallogr. 32 (1976) 751–767. [21] M. Ferhi, K. Horchani-Naifer, S. Hraiech, M. Férid, Y. Guyot, G. Boulon, Near infrared and charge transfer luminescence of trivalent ytterbium in KLa(PO3)4 powders, Optic Commun. 285 (2012) 2874–2878. [22] M. Ferhi, K. Horchani-Naifer, M. Férid, Spectroscopic properties of Eu3+-doped KLa (PO3)4 and LiLa(PO3)4 powders, Opt. Mater. 34 (2011) 12–18. [23] H.R. Abd, Z. Hassan, N.M. Ahmed, M.A. Almessiere, A.F. Omar, F.H. Alsultany, F.A. Sabah, U.S. Osman, Effect of annealing time of YAG:Ce3+ phosphor on white light chromaticity values, J. Electron. Mater. 47 (2018) 1638–1646. [24] D. Petrov, B. Angelov, V. Lovchinov, Magnetic and XPS studies of lithium lanthanide tetraphosphates LiLnP4O12 (Ln = Nd, Gd, Er), J. Rare Earths 31 (2013) 485–489. [25] B. Ramesh, G. Devarajulu, B. Deva Prasad Raju, G. Bhaskar Kumar, G.R. Dillip, A.N. Banerjee, S.W. Joo, Determination of strain, site occupancy, photoluminescent, and thermoluminescent-trapping parameters of Sm3+-doped NaSrB5O9 microstructures, Ceram. Int. 42 (2016) 1234–1245. [26] M.T. Rinke, H. Eckert, The mixed network former effect in glasses: solid state NMR and XPS structural studies of the glass system (Na2O)x(BPO4)1-x, Phys. Chem. Chem. Phys. 13 (2011) 6552–6565. [27] E. Swatsitang, A. Karaphun, S. Phokha, S. Hunpratub, T. Putjuso, Investigation of structural, morphological, optical, and magnetic properties of Sm-doped LaFeO3 nanopowders prepared by sol–gel method, J. Sol. Gel Sci. Technol. 81 (2017) 483–492. [28] P.V. Bernhardt, B.M. Flanagan, M.J. Riley, B.J. Wood, An XPS study of an isomorphous trivalent lanthanoid series, J. Electron. Spectrosc. Relat. Phenom. 124 (2002) 73–77. [29] P. Kumar, V. Sharma, A. Sarwa, A. Kumar, Surbhi, R. Goyal, K. Sachdev, S. Annapoorni, K. Asokan, D. Kanjilal, Understanding the origin of ferromagnetism in Er-doped ZnO system, RSC Adv. 6 (2016) 89242–89249. [30] M. Beltaif, T. Koubaa, T. Kallel, M. Dammak, K. Guidara, K. Khirouni, Synthesis, structure and optical properties of erbium-doped lithium barium phosphate, Solid State Sci. 90 (2019) 21–28. [31] V. Singh, M. Seshadri, N. Singh, M. Mohapatra, Radiative properties of Er3+ doped and Er3+/Yb3+ co-doped Sr3Al2O6 phosphors: exploring the usefulness as a phosphor material, J. Mater. Sci. Mater. Electron. 30 (2019) 2927–2934. [32] J. Chékir-Mzali, K. Horchani-Naifer, M. Férid, Synthesis of Er3+-doped Na3La(PO4)2 micro-powders and photoluminescence properties, Superlattice. Microst. 85 (2015) 445–453. [33] A. Sarakovskis, G. Krieke, G. Doke, J. Grube, L. Grinberga, M. Springis, Comprehensive study on different crystal field environments in highly efficient NaLaF4:Er3+ upconversion phosphor, Opt. Mater. 39 (2015) 90–96. [34] J. Zhao, D. Zhao, Y.L. Xue, Q. Zhong, S.R. Zhang, B.Z. Liu, Novel tantalum phosphate Na13Sr2Ta2(PO4)9:synthesis, crystal structure, DFT calculations and Dy3+activated fluorescence performance, Acta Crystallogr. C 74 (2018) 1045–1052. [35] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A 28 (1968) 444–445. [36] L.G. Van Uitert, Characterization of energy transfer interactions between rare earth ions, J. Electrochem. Soc. 114 (1967) 1048–1053. [37] E. Fred Schubert, Light-Emitting Diodes, second ed., Cambridge University Press, Cambridge, 2006.
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Short communication
Understanding the downconversion photoluminescence of NaLaP4O12:Er3+ phosphor
T
Yang Zhoua, Xinchen Gea, Zhuohui Zhanga, Wenjie Luoa, Hang Xua, Wufu Lia, Jing Zhua,b,∗ a b
School of Materials Science and Engineering, Yunnan University, Kunming, Yunnan, 650091, PR China Yunnan Key Laboratory for Micro/nano Materials & Technology, Kunming, Yunnan, 650091, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metaphosphate Phosphor Photoluminescence White LEDs
We have employed the solid-state chemical reaction to construct series of trivalent erbium ions activated NaLaP4O12 (abbreviated as: NLP) phosphors. The structural investigation indicates that NLP:Er3+ crystalline products are purely monoclinic phase with micron particles. Trivalent erbium ions with the substitution for lanthanum are successfully inserted into NLP host lattice. The UV–Vis diffuse reflectance absorption performance, doping Er3+ content-dependent downconversion photoluminescence, content quenching mechanism were reported in this paper. Upon near UV (378 nm) excitation, yellow/yellow-green visible light with the dominant 4S3/2 → 4I15/2 emission of trivalent erbium is displayed. Yellow-green emitting NLP:2%Er3+ sample has the maximum intensity and main luminescent peaks at 541 and 549 nm. The content quenching is related to the dipolar interaction. Additionally, the detailed investigation on CIE chromaticity coordinates, CP and CCT reveals that the title product has promising application in the field of white LEDs excited by near UV light.
1. Introduction Sodium lanthanum metaphosphate NaLaP4O12 (NLP) materials was early reported [1,2]. It is a monoclinic phase with P21/n space group, and the basic structural units are [(PO3)4]4– chains and LaO8 polyhedron. However, in recent years, considerable research has supported the importance of NLP-based phosphors activated by rare-earth (RE) ions (RE = Ce, Pr, Nd, Eu, Tb and Dy) [3–6]. NLP has the characteristics of facile synthesis, environmental protection as well as favorable crystalline field for typical narrow emission lines of RE activator ions. For example, Liu et al. developed NLP:RE3+ (RE3+ = Eu3+, Tb3+) phosphors via the solid-state chemical reaction [6]. Owing to the large distance and weak interaction between RE ions in NLP host lattice, content quenching phenomenon of Eu3+ or Tb3+ is not observed in NLP. Upon vacuum ultraviolet (UV) light (172 nm) excitation, Eu3+/ Tb3+-codoped NLP phosphors display the Tb3+ → Eu3+ energy transfer and Eu3+ content-dependent tunable visible light emitting from yellowish-green to reddish-orange, indicating the promising applications in field emission display (FED) and plasma display panel (PDP). However, we have not checked the optical report on Er3+-activated NLP materials to this day. Among RE ions, trivalent erbium ion is an effective activator for downconversion (DC) and upconversion (UC) photoluminescence, because its abundant 4f emission lines cover the UV, visible and near ∗
infrared (NIR) light band. Several illustrations (LaBMoO6:Er3+ [7], NaLuCrF4:Er3+ [8], Sr2Gd8(SiO4)6O2:Er3+ [9], SrGdGa3O7:Er3+ [10], Y2Mo4O15:Er3+ [11] and Gd2O2S:Er3+ [12]) show the multifunctional applications of Er3+-activated DC and UC emitting materials. For instance, BaTiO3:Er3+ green emitting phosphor reported [13] can be promisingly applied to the display and temperature sensing domains. Upon excitation at both 380 nm UV light for DC and 980 nm laser for UC, there is green luminescence performance with emission peaks at 550 (4S3/2 → 4I15/2 of Er3+) and 525 (2H11/2 → 4I15/2). Another example is that Er3+-activated telluroborate glass may be employed as potential materials for broadband optical amplifier and green laser [14]. However, compared with UC photoluminescence research, there is little research on DC photoluminescence of trivalent erbium. In aspect of white light emitting diodes (LEDs), inserting RE ions into host matrix is a key design strategy for different visible light emittings [15–19]. Our research aim is to acquire new phosphor with superior luminescence, facile synthesis and environmental protection. Herein, in this paper, we constructed a series of NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples via the solid-state chemical reaction. We have systematically researched the crystalline phase composition, particle morphology, ionic states, absorption performance, Er3+ content-dependent DC photoluminescence behavior, fluorescent quenching mechanism, color coordinates, correlated color temperature and color purity. Our work shows that yellow-green emitting NLP:2%
Corresponding author. School of Materials Science and engineering, Yunnan University, Kunming, Yunnan, 650091, PR China. E-mail address: [email protected] (J. Zhu).
https://doi.org/10.1016/j.ceramint.2019.06.268 Received 2 June 2019; Received in revised form 24 June 2019; Accepted 26 June 2019 Available online 27 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Er3+ powder is a prospective phosphor component for white LEDs excited by near UV light. 2. Experimental details 2.1. Synthesis Series of targeted NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples were constructed via the solid-state chemical reaction. Analytical reagents Na2CO3, La2O3, NH4H2PO4 and Er2O3 (99.99%) were purchased as starting materials. The solid-state chemical reaction is illustrated as following:
(1/2)Na2CO3 + [(1 − x)/2]La2O3 + 4NH 4 H2 PO4 + (x/2)Er2O3 700 °C ⇒ NaLa1 −x Erx P4 O12 + (1/2)CO2 + 4NH3 + 6H2 O
(1)
The synthesis steps can be detailedly described as below: Firstly, based on the stoichiometric ratio of chemical formula, all starting materials were accurately weighed, fully mixed and ground in an agate mortar. Then the mixture was packed into a corundum crucible and preheated at 400 °C for 3 h in the air atmosphere. The preheated mixture was again thoroughly ground. The next step was to raise the temperature to 700 °C and sinter for 7 days by several intermediate grindings. In the end, the targeted sample was cooled to room temperature, pulverized again into white powder and collected for the following characterizations. 2.2. Characterizations For all as-prepared samples, after drying at 333 K, we have performed all measurements at room temperature. Powder X-ray Diffraction (PXRD) for the phase purity identification was performed using a diffractometer (Rigaku D/Max-3B) with Cu Kα radiation (λ = 1.54056 Å), 40 kV and 40 mA. A spectrometer (Thermo Scientific Nicolet iS 10) was employed for fourier transform infrared (FT-IR) analysis (The background is KBr pellet). The surface morphology information was obtained by a scanning electron microscopy (SEM) (Quanta 200 FEG), and the elementary composition was analyzed using the microscope with an energy dispersive X-ray (EDX) detector. An electron spectrometer (Thermo fisher Scientific K-Alpha+) with Mono Al Kα (1486.6 eV) excitation was used for X-ray photoelectron spectroscopy (XPS) analysis. A UV/VIS spectrophotometer (SHIMADZU UV2600) was employed for UV–Vis diffuse reflectance spectrum (DRS). A spectrophotometer (Edinburgh, FLS920) with xenon lamp excitation was used for photoluminescence excitation (PLE) and photoluminescence (PL) data. 3. Results and discussion 3.1. PXRD and FT-IR studies
Fig. 1. PXRD patterns for NLP as the function of Er3+ content (x).
trivalent erbium ion with eight-coordinated environment is close to that of trivalent lanthanum. In addition, the ionic radii of sodium and phosphorus are much smaller than the above radius data [20]. Hence, doping trivalent erbium replaces lanthanum with the same oxidation state, and the crystalline structure of NLP-based samples is unchanged in spite of the introduction of Er3+ ions. The FT-IR spectra for pure NLP and NLP:2%Er3+ samples are illustrated in Fig. 2. The two FT-IR curves display a series of similar vibration peaks (including relative intensities, positions and shapes) between 1400 and 400 cm−1. The vibration bands/lines in the 1310–1150 and 1150–1000 cm−1 regions result from the antisymmetric and symmetric stretching modes (νas and νs) for (O–P–O)– groups in the same order. The vibration bands/lines in the regions of 1000–850 and 850–620 cm−1 are associated with the νas and νs modes for (P–O–P) bridges in the same order. In the low frequency range below 620 cm−1, the vibration patterns result from the bending modes (δ) of (O–P–O)– groups and (P–O–P) bridges. However, it is difficult that the antisymmetric and symmetric bending modes (δas and δs) are clearly distinguished owing to the overlap with external vibrations [21,22]. Therefore, for Er3+-doped samples, [(PO3)4]4– chain in NLP host matrix is not destroyed. The FT-IR investigation is well comparable to the PXRD results (Fig. 1). 3.2. Surface morphology and elemental analysis Fig. 3 is the SEM micrograph for NLP:2%Er3+ powder sample. The crystalline particles are on the uneven micron scale. The observed nonregular shape results from the grain aggregation. This aggregation phenomenon is widespreadly present in the solid-state chemical reaction. The micron scale of as-synthesized particles is similar to that of commercial phosphor for LEDs [23]. The inset of Fig. 3 is the EDX
The phase purity of all as-prepared NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples was checked by PXRD technology, as depicted in Fig. 1. The bottom of Fig. 1 is the simulated PXRD reflection of NLP compound via the Visualizer software based on the structural data (ICSD code: 415682) of crystallographic information file (CIF) reported previously [1]. The diffraction data of NLP compound is also obtained and marked as JCPDS 36–0113. So the comparison between the experimental and simulated diffraction data/JCPDS 36–0113 was carried out, and all PXRD patterns stay the same in the measured range from 10 to 90°. All the as-synthesized NLP:Er3+ powder samples crystallize in the monoclinic structure of NLP host matrix (lattice constants: a = 7.2655, b = 13.1952, c = 10.076 Å, β = 90.38°, V = 965.96 Å3 and Z = 4) [1]. From the view of the crystalline coordination in NLP, the effective radius of trivalent lanthanum ion with eight-coordinated environment is 1.16 Å [20]. The effective radius (1.004 Å) [20] of 20989
Fig. 2. FT-IR spectra for NLP host and NLP:2%Er3+.
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3.4. Downconversion photoluminescence properties of erbium
Fig. 3. SEM image for a typical sample NLP:2%Er3+. The inset shows the EDX spectrum.
spectrum for NLP:2%Er3+ powder sample. Typical peaks for erbium, lanthanum, phosphorus and oxygen are observed without impurity source. The conductive adhesive employed for the EDX measurement results in the presence of carbon peak. The chemical valence states for these ions are investigated by XPS study (Fig. 4). The reference value for the calibration of XPS data is the binding energy (284.4 eV) of C 1s peak. From the wide scan XPS spectrum for NLP:2%Er3+ powder sample (Fig. 4(a)), there are characteristic peaks for Er 3d, Er 4d, O 1s, P 2p, La 3d and Na 1s. It keeps consistent with the elemental composition in the inset of Fig. 3. The high resolution scan XPS spectrum (Fig. 4(b)) for P 2p consists of two peaks as P 2p1/2 (134.62 eV) and P 2p3/2 (133.98 eV), originating from the O–P – O bond of [(PO3)4]4– chains in NLP host matrix. The oxidation state of phosphorus ion is +5 [24]. For O 1s of O2− ion (Fig. 4(c)), the lattice oxygen (including P–O – P, Na–O, La–O or Er–O bonds) is associated with O1 (531.12 eV) and O2 (532.89 eV) peaks, and chemisorbed oxygen gives rise to weak O3 peak at 535.07 eV [25]. The binding energy (1070.56 eV) of Na 1s peak supports the fact that the oxidation state for sodium ion is +1 in the Na–O bond (Fig. 4(d)) [26]. For La 3d, La 3d5/2 results in two peaks at 834.59 and 837.59 eV, whereas La 3d3/2 contributes to two peaks at 851.4 and 854.9 eV. The result shows that the oxidation state of lanthanum ion is +3 (Fig. 4(e)) [27]. For Er 4d (Fig. 4(f)), the peak at 170.76 eV corresponds to Er 4d, suggesting the +3 oxidation state of erbium. It keeps consistent with the binding energy of trivalent erbium in erbium oxide [28,29]. The XPS result supports the fact that trivalent erbium ions are successfully introduced into NLP host lattice.
3.3. Absorption performance The room temperature UV–Vis DRS for NLP:xEr3+ (x = 0, 0.5, 1, 2, 3, 4, 5 mol%) powder samples are presented in Fig. 5. For pure NLP host matrix, the cutoff edge of UV absorption originating from the host is located at around 310 nm, and visible light absorption is not observed. For all Er3+-doped samples, the magnified absorption part between 350 and 850 nm is the inset of Fig. 5. Some UV–Vis absorption peaks with identical position and shape are centered at 363 (4I15/2 → 4G9/2), 378 (4I15/2 → 4G11/2), 407 (4I15/2 → 2H9/2), 442 (4I15/2 → 4F3/2), 451 (4I15/ 4 4 4 4 2 4 2 → F5/2), 488 ( I15/2 → F7/2), 522 ( I15/2 → H11/2), 541 ( I15/2 → 4 4 4 4 4 S3/2), 652 ( I15/2 → F9/2), 663 ( I15/2 → F9/2) and 800 nm (4I15/2 → 4 I9/2), which result from the intrinsic 4f – 4f transitions of Er3+ [10,30]. The 4I15/2 → 4G11/2 (378 nm) absorption is the strongest. The absorption performance also suggests that the Er3+ activator ions are successfully inserted into NLP host matrix.
For NLP:2%Er3+ powder sample, the PLE (λem = 541 nm) and PL (λex = 378 nm) spectra are illustrated in Fig. 6. For PLE (λem = 541 nm) (Fig. 6(a)), there are some excitation peaks at 363 (4I15/2 → 4G9/2), 378 (4I15/2 → 4G11/2), 405 (4I15/2 → 2H9/2), 442 (4I15/ 4 4 4 4 4 2 → F3/2), 451 ( I15/2 → F5/2) and 468 nm ( I15/2 → F7/2), corresponding to the characteristic 4f – 4f transitions for Er3+ ion [31]. The obtained excitation information keeps consistent with the UV–Vis absorption performance (Fig. 5). The predominant excitation line is centered at 378 nm in near-UV region, indicating that the commercial nearUV LED chip can irradiate the synthesized powder sample for downconversion photoluminescence performance. For the PL (λex = 378 nm) spectrum (Fig. 6(b)), the typical 4f – 4f transitions of Er3+ ion results in three emission components [32,33]. The first emission part consists of several weak peaks at 450, 457, 462, 467, 473, 481 and 492 nm, which are related to the intrinsic transitions of trivalent erbium from the excited state 4F7/2 to ground state 4I15/2. The second includes two peaks at 523 and 531 nm, which result from the 2H11/2 → 4I15/2 transitions. The third (4S3/2 → 4I15/2) is predominant, and strong peaks are located at 541 and 549 nm. Fig. 7 depicts the Er3+-doped content-dependent PL (λex = 378 nm) spectra of NLP:Er3+ powder samples. All the PL spectra exhibit identical emitting peaks. For the strongest 541 nm emission (the inset of Fig. 7), Er3+-doped content-dependent emission intensity shows that the emission intensity increases when enlarging the Er3+-doped content (< 2 mol%), whereas it decreases when the doping content is larger than 2 mol%. Hence, NLP:2%Er3+ sample has the maximal 541 nm emission intensity. The Er3+-doped content quenching phenomenon gives rise to the PL result [34]. For NLP:Er3+ powder samples, to understand the content quenching mechanism, the formula (marked as (2)) reported by Blasse [35] is used for computing the critical energy transfer distance (Rc) for erbium ions.
R c ≈ 2(3V/4πχ cN)1/3
(2)
where χc is the Er3+-doped critical content, V is the volume per unit cell, and N is the substitution position number inside each cell. According to the structural data (ICSD code: 415682) of crystallographic information file (CIF) for NLP crystal reported previously [1], V and N is 965.96 Å3 and 4 in the same order. χc is equal to 2%. The calculated Rc is 28.5 Å. For the exchange interaction, the Rc is about 5 Å. Therefore, the content quenching phenomenon for NLP:Er3+ is mainly ascribed to the multipolar – multipolar interaction between Er3+ ions. We further research the multipolar energy transfer mode by the below equation [36]:
I/ χ= K[1 + β(χ)θ/3]−1
(3)
where K and β are constants, χ is the Er -doped content, and the emission intensity is marked as I. The different values of θ correspond to the different interaction mechanism. Since β(χ)Ɵ/3 is much greater than 1, the above formula (3) can be simplified to the below expression [36]: 3+
log (I/χ) = K, − θlog (χ )/3
(4)
From Fig. 8, the correlation of log (I/χ) and logχ is linear with the slope (−θ/3) value of −1.82. So the obtained θ equals 5.46, which is close to 6. Therefore, the dipole – dipole interaction is the content quenching mechanism of the as-prepared samples. 3.5. CIE chromaticity coordinates, CCT and CP To exactly determine the visible light color, correlated color temperature (CCT) and color purity (CP), we have speculated these performance indexes on the basis of the PL spectra of NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%), as shown in Table 1. The calculated CCT values for all as-synthesized samples range from 4325 to 5124 K. As drawn in Fig. 9, when the doping Er3+ content increases from 0.5 to 2%, CIE
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Fig. 4. Wide scan XPS spectrum (a) for a typical sample NLP:2%Er3+ and XPS spectra for P 2p (b), O 1s (c), Na 1s (d), La 3d (e) and Er 4d (f).
chromaticity coordinates (x, y) change from the yellow to yellow-green light in the standard CIE diagram. The emitting light changes towards the yellow light region when increasing the doping Er3+ content from 3 to 5%. The detailed CP values for all as-synthesized samples have been computed by the below formula [37]:
CP =
(x−xi)2 + ( y− yi)2 / (x d − xi)2 + (yd − yi)2
(5) 3+
Fig. 5. UV–Vis DRS curves for NLP as the function of Er3+ content (x). The inset shows the magnified absorption curves between 350 and 850 nm.
where the CIE chromaticity coordinates of NLP:Er are expressed as (x, y), the color coordinates of the dominant emission (541 nm) are marked as (xd, yd), and the chromaticity coordinates (0.33, 0.33) corresponding to standard white light are marked as (xi, yi). The calculated CP for visible light emitting of NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%) are more than 90%. The above results indicate that the as-prepared yellow-green emitting NLP:2%Er3+ sample is a prospective ingredient for white LEDs excited by near-UV light. 4. Conclusions On conclusion, erbium-activated sodium lanthanum phosphate (NLP) powders were developed using the solid-state chemical reaction. 20991
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Fig. 6. PLE (λem = 541 nm) and PL (λex = 378 nm) spectra for a typical sample NLP:2%Er3+.
Fig. 7. Er3+-doped content-dependent PL (λex = 378 nm) spectra for NLP:xEr3+ samples (x = 0.5, 1, 2, 3, 4, 5 mol%). The inset shows the Er3+ content-dependent PL intensity for 541 nm emission.
Fig. 9. CIE chromaticity image for NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%) samples under 378 nm excitation.
The micro-sized samples are NLP-based monoclinic phases. Yellow/ yellow-green visible light with dominant 4S3/2 → 4I15/2 is acquired upon 378 nm (4I15/2 → 4G11/2) excitation. The 2 mol% content of erbium ions is the optimum proposal for yellow-green emitting. The dipole – dipole interaction is the content quenching mechanism of the as-prepared samples. Yellow-green emitting NLP:2%Er3+ powder with high CP is a prospective phosphor component for white LEDs excited by near-UV light. Fig. 8. The curve of log (I/χ) vs. logχ for NLP:Er3+.
Acknowledgements Table 1 CCT, CIE chromaticity coordinates and CP for NLP:xEr3+ (x = 0.5, 1, 2, 3, 4, 5 mol%) samples under 378 nm excitation. NLP:xSm3+
CCT (K)
CIE (x, y)
CP (%)
x = 0.5% x = 1% x = 2% x = 3% x = 4% x = 5%
4499 4561 5124 4611 4612 4325
(0.3740, (0.3718, (0.3471, (0.3696, (0.3906, (0.3854,
98 92 94 94 96 95
0.4394) 0.4420) 0.4197) 0.4416) 0.4612) 0.4580)
This research was financially supported by the National Natural Science Foundation of China (21761034), the Natural Science Foundation of Yunnan Province (2018FB017), and Undergraduate Innovation and Entrepreneurship Foundation of Yunnan University (201810673044 and 201804002).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.06.268.
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Y. Zhou, et al.
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